This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 11-183546, filed Jun. 29, 1999, the entire contents of which are incorporated herein by reference.
The present invention relates to an image forming apparatus such as a digital copying machine, laser printer, or the like that forms a single electrostatic latent image on a single photosensitive drum by simultaneously exposing/scanning the surface of the photosensitive drum with a plurality of laser beams, and an image forming method used in that apparatus.
Especially, the present invention relates to an improvement in an arrangement used to detect/correct any offset produced in passage position control of laser beams with respect to the photosensitive drum, power control of laser beams, or a circuit section that makes such control in the image forming apparatus or method.
More particularly, the present invention relates to an arrangement that digitally controls the positions or power levels of laser beams using a versatile digital device without using a conventional A/D converter in a high-speed digital copying machine using multi-beams.
In recent years, various digital copying machines that form images by scanning exposure using a laser beam and an electrophotography process have been developed.
Recently, a digital copying machine that uses a multi-beam system, i.e., generates a plurality of laser beams and simultaneously scans in units of a plurality of lines using these laser beams to achieve higher image formation speed has been developed.
Such multi-beam digital copying machine comprises a plurality of semiconductor laser oscillators for generating laser beams, a rotary polygonal mirror such as a polygonal mirror for reflecting the laser beams output from these laser oscillators toward a photosensitive drum and scanning the surface of the photosensitive drum with the laser beams, and an optical system unit mainly constructed by a collimator lens, f-xcex8 lens, and the like.
A beam detector for multi-beams normally has the following arrangement. That is, two sensor outputs responsive to laser beams are amplified, and a differential amplifier detects the output difference between the two sensor outputs. The output difference is integrated by an integrator, and the integration result is converted into digital data by an analog-to-digital converter (to be abbreviated as an A/D converter or ADC hereinafter). The digital integration result (digital data corresponding to the beam detection result) is used in the subsequent digital processes.
The digital copying machine converts the positions and analog power levels (corresponding to the integration result) of multi-laser beams into digital data for digital processes in the passage position control of laser beams with respect to the photosensitive drum or power control of laser beams. In order to implement the digital processes, an A/D converter is used. Generally speaking, the device cost of the A/D converter is inevitably higher than that of a versatile logic IC (comparator, flip-flop, inverter, or the like).
In an actual digital copying machine, 8 to 12 bits are assigned to an image process so as to realize excellent halftoning. For this reason, the digital copying machine uses a high-resolution, multi-bit A/D converter (8 to 12 bits), but such A/D converter is especially expensive. A 16-bit A/D converter is more expensive.
As the processing speed of an optical system and digital processing system (including software processes implemented by a CPU or MPU) increases to meet requirements for high print speed, the operation speed requirements for high print speed, the operation speed (the time required for completing each A/D conversion) of the A/D converter used therein must be increased (shortened). A high-resolution A/D converter that can meet such high-speed requirements is still more expensive. In practice, a high-resolution, high-speed A/D converter is very expensive compared to the total cost of a control circuit system of an image forming apparatus.
It is an object of the present invention to provide an image forming apparatus or method which can implement digital control without using an expensive A/D converter.
In order to achieve the above object, an image forming apparatus according to the present invention that controls the beam passage positions of multi-beams comprises a light source (laser) for producing a plurality of light beams; scan means (galvano mirror, polygonal mirror, polygonal mirror motor, polygonal mirror motor driver) for scanning the plurality of light beams so the light beams pass a predetermined object (photosensitive drum); detection means (beam position detector, beam position detector output processing circuit including integrator, CPU software of main controller) for detecting a passage position of each light beam scanned by the scan means; conversion means for converting an analog amount (integration output Vo) corresponding to each light beam passage position detected by the detection means into a corresponding digital bit on the basis of a predetermined threshold value (Vr in FIG. 9 or WINTHH/WINTHL in FIG. 35); arithmetic means for calculating an amount of change in a path of each of the light beams scanned by the scan means on the basis of the threshold value and the digital bit; light path change means (galvano mirror) for changing the passage position of each of the light beams scanned by the scan means so that the passage position of each of the light beams matches a prescribed position (e.g., an intermediate position between the adjacent or neighboring sensor patterns of SF to SJ shown in FIG. 3); and change means for changing the threshold value (Vr or WINTHH/WINTHL) on the basis of the digital bit.
In order to achieve the above object, an image forming apparatus according to the present invention that controls the beam power levels of multi-beams comprises a light source for producing a plurality of light beams; scan means for scanning the plurality of light beams so the light beams pass a predetermined object; detection means for detecting a light amount of each light beam scanned by the scan means; conversion means for converting an analog amount (integration output Vo) corresponding to each beam light amount detected by the detection means into a corresponding digital bit on the basis of a predetermined threshold value (Vr or WINTHH/WINTHL); light amount change means (laser driver) for changing the light amount of each of the light beams scanned by the scan means so that the light amount of each of the light beams matches a prescribed value; and change means for changing the threshold value (Vr or WINTHH/WINTHL) on the basis of the digital bit.
Note that the conversion means can be formed of a single comparator that makes level comparison of the analog amount (Vo) with the threshold value (Vr) changed by the changing means.
Alternatively, the conversion means can be formed of a window comparator that makes level comparison of the analog amount (Vo) with a pair of threshold values (WINTHH/WINTHL) changed by the changing means.
In order to achieve the above object, an image forming apparatus according to the present invention that controls the beam passage positions and beam power levels of multi-beams comprises a light source for producing a plurality of light beams; scan means for scanning the plurality of light beams so the light beams pass a predetermined object; first detection means for detecting a passage position of each light beam scanned by the scan means; first conversion means for converting an analog amount (integration output Vo) corresponding to each beam passage position detected by the first detection means into a corresponding first digital bit on the basis of a predetermined first threshold value (Vr or WINTHH/WINTHL); arithmetic means for calculating an amount of change in a path of each of the light beams scanned by the scan means on the basis of the first threshold value and the first digital bit; light path change means (galvano mirror) for changing the passage position of each of the light beams scanned by the scan means so that the passage position of each of the light beams matches a prescribed position; first change means for changing the first threshold value (Vr or WINTHH/WINTHL) on the basis of the first digital bit; second detection means for detecting a light amount of each light beam scanned by the scan means; second conversion means for converting an analog amount (integration output Vo) corresponding to each beam light amount detected by the second detection means into a corresponding second digital bit on the basis of a predetermined second threshold value (Vr or WINTHH/WINTHL); light amount change means (laser driver) for changing the light amount of each of the light beams scanned by the scan means so that the light amount of each of the light beams matches a prescribed value; and second change means for changing the second threshold value (Vr or WINTHH/WINTHL) on the basis of the second digital bit.
Note that the first and/or second conversion means can be formed of a single comparator that makes level comparison of the analog amount (Vo) with the threshold value (Vr) changed by the first and/or second changing means.
Alternatively, the first and/or second conversion means can be formed of a window comparator that makes level comparison of the analog amount (Vo) with a pair of threshold values (WINTHH/WINTHL) changed by the first and/or second changing means.
In order to achieve the above object, an image forming apparatus according to the present invention which includes a circuit arrangement that produces an offset upon controlling multi-beams comprises a light source for producing a plurality of light beams; scan means for scanning the plurality of light beams so the light beams pass a predetermined object; detection means for detecting a circuit state (offset voltage generation state; cf. FIG. 7) when a light amount of each light beam scanned by the scan means is substantially zero; offset detection means for detecting an offset amount corresponding to the circuit state detected by the detection means; conversion means for converting an analog amount (integration output Vo) corresponding to the offset amount detected by the offset detection means into a corresponding digital bit on the basis of a predetermined threshold value (WINTHH/WINTHL); and change means for changing the threshold value (WINTHH/WINTHL) on the basis of the digital bit.
Note that a deviation resulting from the detected offset amount can be corrected using the threshold value (WINTHH/WINTHL) which is changed in correspondence with the offset amount.
The conversion means can be formed of a single comparator that makes level comparison of the analog amount (Vo) with the threshold value (Vr) changed by the changing means.
Alternatively, the conversion means can be formed of a window comparator that makes level comparison of the analog amount (Vo) with a pair of threshold values (WINTHH/WINTHL) changed by the changing means.
In order to achieve the above object, an image forming method according to the present invention that controls the beam passage positions of multi-beams comprises the steps of: producing a light beam used in image formation; detecting a passage position of the light beam by scanning the light beam; converting an analog amount (integration output Vo) corresponding to the detected passage position of the light beam into a corresponding digital bit on the basis of a predetermined threshold value (Vr in FIG. 9, or WINTHH/WINTHL in FIG. 35); calculating an amount of change in a path of the light beam on the basis of the threshold value and the digital bit; changing the passage position of the light beam so that the passage position of the light beam matches a prescribed position (e.g., an intermediate position between the adjacent or neighboring sensor patterns of SF to SJ shown in FIG. 3); and changing the threshold value (Vr or WINTHH/WINTHL) on the basis of the digital bit.
In order to achieve the above object, an image forming method according to the present invention that controls the beam power levels of multi-beams comprises the steps of: producing a light beam used in image formation; detecting a light amount of the light beam; converting an analog amount (integration output Vo) corresponding to the detected light amount of the beam into a corresponding digital bit on the basis of a predetermined threshold value (Vr or WINTHH/WINTHL); changing the light amount of the light beam so that the light amount of the light beam matches a prescribed value; and changing the threshold value (Vr or WINTHH/WINTHL) on the basis of the digital bit.
In order to achieve the above object, an image forming method according to the present invention which includes a circuit arrangement that produces an offset upon controlling multi-beams comprises the steps of: producing a light beam used in image formation; detecting a circuit state (offset generation state; cf. FIG. 7) when a light amount of the beam is substantially zero; detecting an offset amount corresponding to the circuit state; converting an analog amount (integration output Vo) corresponding to the detected offset amount into a corresponding digital bit on the basis of a predetermined threshold value (WINTHH/WINTHL); changing the threshold value (WINTHH/WINTHL) on the basis of the digital bit; and recording a final value of the changed threshold value (WINTHH/WINTHL) as data indicating the offset amount.
Note that a deviation resulting from the offset amount can be corrected using the final value of the threshold value recorded as data indicating the offset amount.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.